1. Field of the Invention
The present invention relates to a dimming control device of a lamp and more particularly to a dimmer applied to a high-power load without high heat generated and having switchable power modes.
2. Description of the Related Art
With reference to FIG. 11, a conventional dimming controller 70 connected with various lighting devices 90 is shown. Conventionally, each lighting device 90 needs to be connected to an output terminal of the dimming controller 70 through a corresponding control signal line 72 to receive a dimming command. Besides, each lighting device 90 must be connected to an input power source 76 to receive a required operating voltage through a corresponding power line 74. To dim the lighting device, a control module inside the lighting device 90 adjusts lighting luminance of the lighting device 90 according to the dimming command.
Basically, it is inconvenient to physically implement the aforementioned approach due to complicated wiring and too much time and effort involved. For instance, if tens of or even hundreds of lighting devices are mounted in a large-scale site, only the wiring work of the power lines 74 is already complicated and tremendous, not to mention enough space required to accommodate the control signal lines 72. Besides, connecting the control signal lines 72 is also another complicated task to do. When the power lines 74 are inadvertently connected with the control signal lines 72, the entire lighting system could be ruined.
The disadvantage of the conventional dimming controller 70 connected with the lighting devices in FIG. 11 is improved by incorporating the control signal lines 72 into the power lines 74 as shown in FIG. 12. The dimming controller 80 has a rectification circuit 82 and a power switch 84 therein. The rectification circuit 82 converts received AC power into full-wave DC power. The dimming controller 80 outputs the full-wave DC power through a first output terminal 801 and a second output terminal 802. The lighting devices 90 are connected in parallel between the first output terminal 801 and the second output terminal 802. The power switch 84 is connected between the second output terminal 802 and the ground in series.
To constantly convert the AC power into the full-wave DC power, the rectification circuit 82 of the dimming controller 80 needs to be always maintained at an operating state. With reference to FIG. 13(A), for sake of making a power supply loop function, a high potential is inputted to an input terminal of the power switch 84 inside the dimming controller 80 so that the power switch 84 is continuously maintained at a turn-on state and current can flow to the ground through the power switch 84. Meanwhile, the dimming controller 80 outputs the full-wave DC power as shown in FIG. 13(C). Hence, the power supply loop is formed by sequentially and serially connecting the rectification circuit 82, the lighting devices 90 and the power switch 84 and looping back to the rectification circuit 82. The power supply loop is the same as the power lines 74 in FIG. 11 in terms of the function of supplying power to the lighting devices 90. The function of the control signal lines 72 in FIG. 11 is implemented by injecting pulse signals of dimming commands as shown in FIG. 13(B) to an input terminal of the power switch 84 to rapidly turn the power switch 84 on and off, so that the full-wave DC power outputted from the rectification circuit 82 is controlled to rapidly turn on and off Such rapid turn-on and turn-off enables the full-wave DC power outputted from the rectification circuit 82 to contain pulses therein having a waveform as shown in FIG. 13(D). When the full-wave DC power is transmitted to the lighting devices 90, the lighting devices 90 further decode the dimming commands of the pulses in the full-wave DC power and perform dimming operation according to the dimming commands.
According to circuit operation, no matter whether users control the dimming controller 80 to adjust brightness of the lighting devices 90 or not, current constantly passes through the rectification circuit 82 and the power switch 84 inside the dimming controller 80 when the lighting devices 90 are normally lit. In other words, the dimming controller 80 continuously consumes power.
Consequently, the circuit in FIG. 12 is good for a light load using a small power and fails to be applicable to a high-power output. For example, under the condition that the input AC power is 110V, twenty 5 W lighting devices 90 are to be controlled, an overall power required by the lighting devices 90 is 100 W, an operating current under the 110V AC power is approximately 0.9 A, and the power switch is composed of an n-MOSFET and its turn-on resistance is 0.5Ω, then
(1) a turn-on voltage of the diode of the rectification circuit 82 is 1V and the consumed power of the rectification circuit 82 is (1V+1V)×0.9=1.8 W because two diodes are on each path through which current flows; and
(2) the power consumed by the n-MOSFET power switch 84 is 0.9 A×0.9 A×0.5Ω=0.4 W.
To such low-power load, the rectification circuit 82 and the power switch 84 can still be normally operated as long as heat generated therefrom is slightly dissipated.
If the original twenty 5 W lighting devices 90 are replaced by a 44 W LED grid light, the overall consumed power is 880 W, an operating power under the 110V AC power is approximately 8 A, and the power switch is composed of n-MOSFET and its turn-on resistance is 0.5Ω, then
(1) the power consumed by the rectification circuit 82 is (1V+1V)×8 A=16 W; and
(2) the power consumed by the n-MOSFET power switch is 8 A×8 A×0.5Ω=32 W.
To such high power load, besides a large-scale heat sink, a fan is also needed to achieve the intended heat-dissipating effect. Up to here, commercial value and feasibility of such product has already gone.
Moreover, when the rectification circuit 82 is operated above 100° C., the rated current thereof starts dropping. For example, the rated current may drop to 5 A from the original 10 A. The lower withstand current rating may lead to direct burn-out of the rectification circuit 82.